All ducts systems require access panels of one type or another to enable the inspection of the interior of the ducts, to allow the cleaning of the duct surfaces and to maintain any equipment such as fire dampers, volume dampers, etc. that may be installed in the duct.
Some ducts, such as grease ducts, contain a grease residue, and are required to be cleaned so as not to pose a fire hazard. The NFPA 96 standard requires that access panels large enough to permit thorough cleaning be provided at 12 ft. intervals in the duct. While many systems when they are designed and installed meet the NFPA requirement, in most cases it is not until the system is to be cleaned that it is discovered that due to the duct size, system design and other modifications done to the surrounding areas after installation, additional access panels are needed to allow proper cleaning of the duct. Since grease is extremely flammable, it is not advisable to weld new access panels on ducts that have been exposed to grease, and therefore other techniques are required to retrofit access panels.
One known approach involves using an access door assembly comprising a mounting and support frame, a releasably secured cover, a seal interposed between the cover and mounting and support frame. The access door assembly is held in position and secured to the duct by support studs. Such an approach utilizing a support/mounting frame suffers from a number of disadvantages as discussed below.
First, the utilization of a mounting and support frame limits the locations on the duct where the access door can be installed, and such an arrangement is not modifiable in the field. Furthermore, the frame prevents installation on ducts with other than flat surfaces thereby eliminating use on ducts with non-flat contours. The frame also prevents the shape of the door from being modified in the field to enable installation close to obstructions such as building members or other building services.
Second, the mounting and support frame when, installed inside a duct creates a surface raised above the normal duct interior causing grease to accumulate around the frame itself.
Third, by having the support studs protrude outwardly from the duct surface, it is not possible to install the access door into a duct with non-flat contours Without creating elongated holes in the duct equal to or greater than the length of the support stud. Such elongated holes could protrude beyond the edge of the access door thereby creating leakage.
Fourth, the support studs protrude outwardly from the duct surface and as such causing a potential safety hazard to the personnel inspecting or cleaning the ducts. The protruding support studs can cause lacerations, bruising and even lead to falls from ladders as a result of clothing or equipment catching on the protruding fasteners.
Accordingly, there remains a need for an access door assembly which overcomes the perceived shortcomings of the prior art.